Field of the Invention
The invention relates to a circuit configuration for line adaptation and echo suppression including a balance filter which is triggered by transmission signals and has output signals that are linked through a subtractor to a reception signal, an impedance adaptation filter which is triggered by the reception signal and has an output signal that is linked through an adder to the transmission signal, an analog/digital converter for converting analog reception signals into digital reception signals, and a digital/analog converter for converting digital transmission signals into analog transmission signals.
One such circuit configuration is known, for instance, from an article by D. Vogel, E. Schmid, J. Reisinger and L. Lerach, entitled "A Signal-Processing Codec Filter for PCM Applications", Siemens Forschungs-und Entwicklungsberichte [Siemens Research and Development Reports] 15 (1986), No. 5, Berlin, pp. 253-258. In that configuration, an analog/digital converter located in the reception path and a digital/analog converter located in the transmission path are provided. An impedance adaptation filter (Z filter) and a balance filter (B filter) are used on the digital side. The impedance adaptation filter picks up digital reception signals at the output of the analog/digital converter and filters them. The output signal of the impedance adaptation filter, which is linked to the transmission signal, is delivered to the digital/analog converter through the use of the adder. The balance filter is likewise constructed digitally and is accordingly triggered by the digital transmission signal. The output signal of the balance filter is linked through the subtractor (or a further adder) to the digital reception signal.
Circuit configurations of the kind referred to at the outset are used particularly in electronic two-wire/four-wire converters, but echo terms which can occur therein depending on the line length under some circumstances are multiple times greater than the received useful signal. The intent is to filter out those echo terms through the use of the balance filter. To that end, the echo path is simulated by the balance filter. The echo terms are eliminated by the ensuing subtraction of output signals from the reception signal. However, in that way the deletion of only linear terms can be attained. If the digital/analog converter and/or the analog/digital converter produces harmonics because of its nonlinearities, then those harmonics pass unfiltered into the reception path and thus reduce the attainable signal-to-noise ratio.